This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-097851, filed Apr. 5, 1999, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a planar type radiation detector installed in an image generation apparatus such as an X-ray diagnosis apparatus using radiation.
2. Discussion of Background
The past mainstream was X-ray inspection using a film. However, the recent remarkable improvement in space and energy resolution associated with a planar type radiation detector allows rapid shift from X-ray inspection using only a film to digital X-ray inspection. In the field of digital X-ray inspection as well, a conventional scheme of combining a bulky image intensifier and a TV camera is being replaced with a scheme using a planar type radiation detector that is a lightweight and compact low-profile device excellent in spatial and energy resolution.
FIG. 1 shows a typical example of a film cassette 100 setting an X-ray film 101 and an intensifying screen 104 between sponges 102.
Most components of an incident X-ray energy are absorbed by the film and the screen. However, the remaining energy components pass through the film, and the screen. A lead plate 103 is bonded to the rear plane of the sponge 102 to prevent the transmission X-ray (to be referred to as a first transmission X-ray hereinafter) from externally leaking or attenuate the transmission X-ray to a safety standard value.
However, this lead plate 103 poses the following problem. The lead plate 103 is designed primarily aiming at prevention or reduction of the leakage X-ray. On the other hand, an X-ray transmitting the film 101, the screen 104, and sponges 102 can reflect from the lead plate 103. In short, a backscattering ray is generated from the lead plate 103. This backscattering ray may generate charges in the conversion elements. The charges generated by the backscattering ray become noise that deteriorates the image quality.
More specifically, when the material, thickness and/or shape of the shielding plate are so designed as to increase the radiation absorbed dose, the scattered radiation dose also becomes large accordingly. Hence, the larger the radiation absorbed dose is set to reduce the leakage X-ray, the larger the scattered radiation dose becomes to result in an increase in noise.
The purpose of suppressing leakage X-ray has priority over the purpose of reducing noise. Hence, the purpose of reducing noise is sacrificed.
An X-ray detector disclosed in U.S. Pat. No. 5,777,335 suffers a similar problem.
The X-ray detector disclosed in U.S. Pat. No. 5,777,335 has a phosphor 4 for converting an X-ray into light, a conversion element 1 for converting light into charges, and a supporting plate 2 of the conversion element 1. A supporting plate 3 or lead plate 5 for absorbing and shielding an X-ray is provided behind the supporting plate 2. This X-ray detector disclosed in U.S. Pat. No. 5,777,335 also suffers a problem that the backscattering ray generated by the supporting plate 3 or lead plate 5 generates noise charges in the conversion element 1.
An X-ray detector disclosed in U.S. Pat. No. 5,753,921 employs a scheme of converting an X-ray into light by a converter 12 and then converting the light into charges by a detection array 14, as shown in FIG. 1 of this prior art. Together with a backing 24, a supporting plate 22 of the detection array 14 constructs a support unit 20 for physically supporting the converter 12 and detection array 14. As a characteristic feature of this prior-art invention, an element with an atomic number of 22 or more is contained in a supporting plate 22 of the detection array 14, thereby imparting the absorbing and shielding function to the supporting plate 22.
It is an object of the present invention to suppress a backscattering ray generated by a shielding plate from reaching a charge conversion element in a planar type radiation detector having the shielding plate designed to prevent leakage of radiation or attenuate the leakage radiation more than a standard value.
A radiation detector has conversion elements for converting radiation incident from the front side into charges, an electric charge read out elements arranged on the rear side of the conversion elements, the supporting plate of the electric charge read out elements, and a first plate as a main shielding plate arranged on the rear side of the supporting plate. The first plate effectively prevents the leakage radiation or reduces the leakage radiation to the standard value. The first plate, however, generates a large amount of backscattering ray. When the backscattering ray reaches the conversion elements, noise charges are generated to degrade the image quality. A second plate is characteristically inserted between the first plate and the supporting plate. The second plate is designed to have a radiation absorbed dose that is not zero but much littler than that of the first plate. The second plate can sufficiently shield the relatively weak backscattering ray generated by the first plate from the conversion elements. Since the second plate has a small radiation absorbed dose, the second plate itself rarely generates the backscattering ray. The backscattering ray generated by the second plate, therefore, does not reach the conversion elements to generate noise charges and degrade the image quality.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.